The present invention relates generally to an apparatus and method for improved sealing within bipolar plates used in a fuel cell assembly, and more particularly to the use of a seal formed within the plate where a volume within the seal houses a plug to avoid shunted fluid flow that would otherwise traverse a bead path formed by the seal.
Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
In such a stack, adjacent MEAs are separated from one another by a series of reactant flow channels, typically in the form of a gas impermeable bipolar plate that—in addition to promoting the conveyance of reactants, coolant and byproducts—provides structural support for the MEA, as well as electrical current collection or conveyance and cell-to-cell sealing. In one common form, the channels are of a generally serpentine layout that covers the majority of the opposing generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes the conveyance of one of the reactants to or from the fuel cell, while additional channels (that are fluidly decoupled from the reactant channels) may also be used for coolant delivery. In one configuration, the bipolar plate is itself an assembly formed by securing a pair of thin metal sheets (called half plates) that have the channels stamped or otherwise integrally formed on their surfaces to promote fluid engagement. The various reactant and coolant flowpaths formed by the channels on each side typically convene at a manifold (also referred to herein as a manifold region or manifold area) defined on one or more opposing edges of the plate. Examples of all of these features—as well as a typical construction of such bipolar plate assemblies that may be used in PEM fuel cells—are shown and described in commonly-owned U.S. Pat. Nos. 5,776,624 and 8,679,697, the contents of which are hereby incorporated by reference.
In a typical bipolar plate construction, a single plate assembly is formed from individual stamped plate layers that are welded together to form a laminated structure with the various fluid passages, sealing surfaces, support structures and electrically conductive surfaces. Historically, the sealing function around the plate manifolds and active area is achieved in one way through the use of separate gasket or seal assemblies where the underlying plate acts as a carrier, while in another way through cure-in-place (CIP) sealing materials placed between the adjacent plate layers during the assembly process. The CIP approach is costly and often requires long manufacturing cycle times to properly cure the seal materials, while both are only suitable for low volume applications, where high manufacturing and material costs could be tolerated.
Unfortunately, commercial automotive fuel cell applications require high volume manufacturing solutions that can produce 10,000 to 100,000 fuel cell stacks per year (each with roughly 300 to 400 cells per stack). Given that each cell requires a bipolar plate assembly on each side of the MEA, even low volume production would require more than 3 million plates be made. The above sealing approaches would be a cost-prohibitive way to achieve high volume bipolar plate production.
To overcome some of the cost and manufacturing issues related to the CIP or the discrete seal and carrier assembly approaches, the Assignee of the present invention has developed an approach for bipolar plate sealing where stamped metal bead seals (MBSs, also referred to herein more simply as “seals”) are used to establish cell-to-cell sealing. While such a configuration is more compatible with the high-volume production needs mentioned above, the metal-to-metal connection makes it difficult to ensure that additional leakage paths aren't introduced where coolant or other fluids can fill the channels formed by the MBS; such channel filling would lead to the coolant or other fluids to bypass the preferred route through the cell by shunting the coolant directly to the discharge side of the plate through the volumetric space formed along the length of the metal bead seals. Such shunting could lead to parasitic losses to the cooling system pump (thereby necessitating larger, less efficient pumps), as well as lead to higher cell operating temperatures (and a more rapid deterioration of the stack). While pre-formed, sealable so-called blind plugs may be used during bi-polar plate assembly and welding operations in order to fill and seal a channel, the placement of the plug requires a high degree of precision that risks miss-installation of the plug. Moreover, such an approach is not visible for quality inspections.